The present invention pertains to a driver circuit that can drive an electroluminescent (referred to as xe2x80x9cELxe2x80x9d hereinafter) panel to light a panel. More specifically, the present invention pertains to an EL driver circuit that can automatically detect the drop in the luminous brightness caused by changes in the EL panel over time and is able to adjust the driving voltage of the EL panel to recover the desired luminous brightness.
The present inventors have invented an EL driver circuit which uses a parallel resonant circuit formed by an autotransformer and an EL panel to drive and light the EL panel. The details are described in Japanese Kokai Patent Application No. Hei 8[1996]-168262.
EL driver circuits generally use a parallel resonant circuit that is driven with a sinusoidal ac driving signal, which causes the EL panel to emit light at a high efficiency. However, in order to obtain the desired effect, the wire size of the autotransformer must be enlarged and a large core must be used. Consequently, the device cannot be miniaturized, leading to a high cost.
Although an EL panel can effectively form a capacitor, it is known that the capacitance will drop over time. In this case, since the EL panel forms a parallel resonant circuit together with the autotransformer, the resonant frequency of the resonant circuit will rise automatically as the capacitance of the EL panel drops. As a result, the number of times that the EL panel is lit per unit time is increased. Consequently, the brightness is recovered to a certain degree. However, when the capacitance of the EL panel decreases, the current level of the driving signal of the EL panel drops, and the luminous brightness becomes low. Therefore, the luminous brightness of the EL panel cannot be fully recovered by simply increasing the number of times that the panel is lit per unit time as a result of increase in the resonant frequency.
An EL driver circuit having high luminous efficiency, small size, and low cost is provided. The EL driver circuit is configured to automatically detect changes in the EL panel occurring over time and to recover the luminous brightness level. In addition, the EL driver circuit can detect the decrease in the capacitance of the EL panel and is able to raise the voltage level of the driving signal of the EL panel.
The EL driver circuit includes a push-pull driver, which has an output end and an input end and is connected between a supply voltage and a first fixed voltage, an EL panel, which can effectively form a capacitor, and a coil which is connected in series to the EL panel; one end of the serial connection of the aforementioned EL panel and coil is connected to the output end of the aforementioned push-pull driver, while the other end is connected to the input end of the aforementioned push-pull driver in a positive feedback manner.
In the EL driver circuit of one embodiment of the invention, an EL panel and a coil are connected to each other in series to form a serial resonant circuit. The serial resonant circuit is connected to a push-pull driver through a positive feedback path, and the whole unit forms an oscillator circuit. When the serial resonant circuit comprising the EL panel and the coil is formed, the driving signal applied to the EL panel becomes an ac signal in the form of a sinusoidal wave free of noise. As a result, the EL panel can be lit at a high efficiency. Also, in this case, since the serial resonant circuit is formed by an EL panel and coil, the coil, especially, can be miniaturized compared with using the parallel resonant circuit described in Japanese Kokai Patent Application No. Hei 8[1996]-168262. Consequently, the entire device can be miniaturized and the cost can be lowered. In addition, since a signal in the form of an almost completely sinusoidal wave can be obtained as the driving signal, excellent luminous properties can be realized.
In accordance with another aspect of the invention, an EL driver circuit includes:
a push-pull driver that is connected between a supply voltage and a first fixed voltage and is configured to output the aforementioned supply voltage in the form of a rectangular wave;
a coil, which is connected to an EL panel that can effectively form a capacitor to constitute a resonant circuit, and which is connected in a loop formed by coupling the push-pull driver and the resonant circuit;
an integrator circuit configured to receive and integrate the driving voltage of the EL panel;
a driving voltage level detecting circuit configured to receive and detect the change in the level of the driving voltage of the EL panel based on the first reference voltage and the integrated voltage obtained by the integrator circuit;
a supply voltage level adjusting circuit configured to adjust the level of the supply voltage supplied to the push-pull driver corresponding to the detection signal sent from the driving voltage level detecting circuit; and
a variable voltage source that can supply an adjusted supply voltage to the push-pull driver corresponding to the adjustment signal.
In the EL driver circuit, the resonant circuit is formed by an EL panel and a coil, and the EL panel is driven and lit by a driving signal in the form of sinusoidal wave. In this case, the voltage level of the driving signal supplied to the resonant circuit can be adjusted, such as increased in response to the changes, especially a decrease, in the capacitance of the EL panel occurring over time. For example, when the capacitance of the EL panel decreases due to the changes occurring over time, the resonant frequency of the resonant circuit formed by the EL panel and the coil rises. As a result, the number of times that the EL panel is lit per unit time is increased. Consequently, the drop in the luminous brightness of the EL panel caused by the decrease in the capacitance of the EL panel can be compensated.
According to a further aspect of the invention, when the capacitance of the EL panel changes, the level of the voltage supplied to the push-pull driver can be adjusted in response to the change in the capacitance, and the voltage level of the signal used for driving the EL panel can be adjusted. For example, when the capacitance of the EL panel decreases, the voltage supplied to the push-pull driver is increased correspondingly. As a result, the voltage level of the ac driving signal in the form of a sinusoidal wave applied to the EL panel is increased. Consequently, the luminous brightness of the EL panel can be prevented from dropping.
In accordance with yet another aspect of the present invention, the resonant circuit comprising the coil and the EL panel is preferably a serial resonant circuit formed by connecting the EL panel to the coil in series. The adjusting mechanism used for adjusting the voltage level of the driving signal of the EL panel is not limited to the serial resonant circuit. For example, it is also possible to use the parallel resonant circuit formed by connecting the coil in parallel with the EL panel described in said Japanese Kokai Patent Application No. Hei 8[1996]-168262.
Consequently, the function of the EL driver circuit of the invention is not simply to increase the frequency of the driving signal in an automatic manner corresponding to the capacitance of the EL panel. Instead, the EL driver circuit can maintain the luminous level on an appropriate level by actively adjusting the voltage level of the driving signal of the EL panel corresponding to the change in the capacitance of the EL panel. Therefore, the EL driver circuit can prevent degradation in the luminous property caused by the deterioration of the EL panel itself, and the service life of the EL panel can be prolonged.